Media communication over bit-rate-limited and error-prone channels, such as packet networks and wireless links, requires both high compression and high error resilience. Important applications within this context include streaming media over the Internet, and streaming wireless media to handheld devices with wireless local area networks (LANs) or with emerging Third Generation (3G) cellular systems. To achieve high compression, most current media compression systems, such as Motion Picture Experts Group (MPEG)-1/2/4 and H.261/3/L, employ motion-compensated prediction between frames to reduce the temporal redundancy, a spatial transform to reduce the spatial redundancy, and entropy encoding to produce the compressed bitstream. These algorithms provide significant compression; however, the compressed signal is much more vulnerable to packet losses when transmitted over error-prone channels such as the Internet. One problem that afflicts media compressed with these encoders is error propagation. Specifically, although the inter-frame prediction provides significant compression, the decoder depends on the reference frame used by the encoder in order to perform correct decoding. A channel error that, for example, results in loss of this reference frame can cause the reconstructed frame at the decoder to be distorted, which in turn can lead to significant error propagation to subsequent frames.
Prior Art FIG. 1 is a block diagram 100 showing an example of a typical system for transmitting media in accordance with conventional art. In the example shown, media data 110 enters an encoder/packetizer 120 where it is compressed, encoded and packetized for transmission. The packets that result can be stored in a storage medium 130 for later transmission over channel 150 as scheduled by scheduler 140.
Scheduler 140 of FIG. 1 schedules the packets for transmission at the appropriate time and transmits them sequentially, in numerical order, to a destined receiver/decoder 160. Receiver/decoder 160 decodes and decompresses the media and makes it available to a viewer over an appropriate viewer medium.
Prior Art FIG. 2A illustrates a stream of packets 200a as transmitted in the conventional art by scheduler 140 of FIG. 1. Each numbered square represents a packet and the numbers represent the sequence number of each packet. Thus, it is shown that the conventional stream of media packets is sequenced in numerical order.
One problem with the conventional art occurs when a number of Consecutively transmitted packets are lost for one reason or another. This type of loss is known as a “burst” loss, or, in the plural as “bursty” losses. Prior Art FIG. 2B illustrates a conventional stream of packets 200b illustrating a burst loss (a loss of several sequential packets) containing frames 2, 3 and 4. Frame 1 is used to estimate the three missing frames. Since frames 3 and 4 are progressively further from frame 1, it follows that frame 1 provides a progressively worse estimate of the frames 3 and 4.